Amphoteric Nafion membrane with tunable cationic and anionic ratios for vanadium redox flow battery prepared via atom transfer radical polymerization

Abstract

Nafion is currently the most stable ion exchange membrane for vanadium redox flow battery (VRB). In order to decrease the high vanadium permeability of Nafion while still keeping its high ionic conductivity and stability, in this study, solution atom transfer radical polymerization (ATRP) method is used with Nafion as initiator to prepare amphoteric Nafion membrane with tunable cationic and anionic ratios. Polymers of [2-(methacryloyloxy)ethyl] trimethylammonium chloride (METAC) and sodium 4-styrenesulfonate (NaSS) are chosen as cationic and anionic monomers, respectively. FT-IR (Fourier transform infrared) and 1H NMR (nuclear magnetic resonance) tests prove that the METAC and NaSS have been successfully bonded on the Nafion molecules. The solution-casted amphoteric Nafion membrane M-1:1 has shown the highest ion selectivity among all the grafted Nafion membranes. The VRB with M-1:1 membrane has shown the highest average energy efficiency of 83.9% at current density of 40–80 mA cm−2, which is 4.2% higher than that of Nafion 212 (79.7%). One hundred cycles dynamic charge-discharge test proves that the M-1:1 membrane possesses enough stability and good discharge capacity retention ability. On-line self-discharge test further proves that the M-1:1 membrane has lower vanadium ions permeation than that of Nafion 212 membrane. All the results prove that ATRP is an effective method for preparation of novel amphoteric materials for VRB application.